There are two types of solids in nature, which differ markedly in their properties. These are amorphous and crystalline bodies. And amorphous bodies do not have an exact melting point; during heating, they gradually soften and then pass into a fluid state. An example of such substances is resin or ordinary plasticine. But the situation is completely different with crystalline substances. They remain in a solid state until a certain temperature, and only after reaching it do these substances melt.

It's all about the structure of such substances. In crystalline solids, the particles of which they are composed are located at certain points. And if you connect them with straight lines, you get some kind of imaginary frame, which is called a crystal lattice. And the types of crystal lattices can be very different. And according to the type of particles from which they are “constructed,” lattices are divided into four types. These are ionic, atomic, molecular and

And at the nodes, accordingly, ions are located, and there is an ionic bond between them. can be either simple (Cl-, Na+) or complex (OH-, SO2-). And these types of crystal lattices may contain some metal hydroxides and oxides, salts and other similar substances. Take, for example, ordinary sodium chloride. It alternates negative chlorine ions and positive sodium ions, which form a cubic crystal lattice. Ionic bonds in such a lattice are very stable and substances “built” according to this principle have fairly high strength and hardness.

There are also types of crystal lattices called atomic ones. Here, the nodes contain atoms between which there is a strong covalent bond. Not many substances have an atomic lattice. These include diamond, as well as crystalline germanium, silicon and boron. There are some more complex substances, which contain and have, respectively, an atomic crystal lattice. These are rock crystal and silica. And in most cases, such substances are very strong, hard and refractory. They are also practically insoluble.

And the molecular types of crystal lattices have a variety of substances. These include frozen water, that is, ordinary ice, “dry ice” - solidified carbon monoxide, as well as solid hydrogen sulfide and hydrogen chloride. Molecular lattices also have many solids. organic compounds. These include sugar, glucose, naphthalene and other similar substances. And the molecules located at the nodes of such a lattice are connected to each other by polar and non-polar chemical bonds. And despite the fact that inside the molecules there are strong covalent bonds between atoms, these molecules themselves are held in the lattice due to very weak intermolecular bonds. Therefore, such substances are quite volatile, melt easily and do not have great hardness.

Well, metals have the most different types crystal lattices. And their nodes can contain both atoms and ions. In this case, atoms can easily turn into ions, giving up their electrons for “common use.” In the same way, ions, having “captured” a free electron, can become atoms. And this lattice determines such properties of metals as plasticity, malleability, thermal and electrical conductivity.

Also, the types of crystal lattices of metals, and other substances, are divided into seven main systems according to the shape of the elementary cells of the lattice. The simplest is the cubic cell. There are also rhombic, tetragonal, hexagonal, rhombohedral, monoclinic and triclinic unit cells that determine the shape of the entire crystal lattice. But in most cases, crystal lattices are more complex than those listed above. This is due to the fact that elementary particles can be located not only in the lattice nodes themselves, but also in its center or on its edges. And among metals, the most common are the following three complex crystal lattices: face-centered cubic, body-centered cubic, and hexagonal close-packed. More physical characteristics metals depend not only on the shape of their crystal lattice, but also on the interatomic distance and other parameters.

Let's talk about solids. Solids can be divided into two large groups: amorphous And crystalline. We will separate them according to the principle of whether there is order or not.

IN amorphous substances the molecules are arranged randomly. There are no patterns in their spatial arrangement. Essentially, amorphous substances are very viscous liquids, so viscous that they are solid.

Hence the name: “a-” – negative particle, “morphe” – form. Amorphous substances include: glass, resins, wax, paraffin, soap.

The lack of order in the arrangement of particles causes physical properties amorphous bodies: they do not have fixed melting points. As they heat up, their viscosity gradually decreases, and they also gradually turn into a liquid state.

In contrast to amorphous substances, there are crystalline substances. The particles of a crystalline substance are spatially ordered. This correct structure of the spatial arrangement of particles in a crystalline substance is called crystal lattice.

Unlike amorphous bodies, crystalline substances have fixed melting points.

Depending on what particles are in lattice nodes, and what connections hold them together differentiate them: molecular, atomic, ionic And metal grates.

Why is it fundamentally important to know what kind of crystal lattice a substance has? What does it define? All. The structure determines how chemical and physical properties of a substance.

The simplest example: DNA. In all organisms on earth, it is built from the same set of structural components: four types of nucleotides. And what a variety of life. This is all determined by structure: the order in which these nucleotides are arranged.

Molecular crystal lattice.

A typical example is water in a solid state (ice). Entire molecules are located at lattice sites. And keep them together intermolecular interactions: hydrogen bonds, van der Waals forces.

These bonds are weak, so the molecular lattice is the most fragile, the melting point of such substances is low.

A good diagnostic sign: if a substance has a liquid or gaseous state under normal conditions and/or has an odor, then most likely this substance has a molecular crystal lattice. After all, the liquid and gaseous states are a consequence of the fact that the molecules on the surface of the crystal do not adhere well (the bonds are weak). And they are “blown away.” This property is called volatility. And the deflated molecules, diffusing in the air, reach our olfactory organs, which is subjectively felt as a smell.

They have a molecular crystal lattice:

1. Some simple substances of non-metals: I 2, P, S (that is, all non-metals that do not have an atomic lattice).
2. Almost all organic substances ( except salts).
3. And as mentioned earlier, substances under normal conditions are liquid, or gaseous (being frozen) and/or odorless (NH 3, O 2, H 2 O, acids, CO 2).

Atomic crystal lattice.

In the nodes of the atomic crystal lattice, in contrast to the molecular one, there are individual atoms. It turns out that the lattice is held together by covalent bonds (after all, they are the ones that bind neutral atoms).

A classic example is the standard of strength and hardness - diamond (according to chemical nature is a simple substance carbon). Contacts: covalent nonpolar, since the lattice is formed only by carbon atoms.

But, for example, in a quartz crystal (the chemical formula of which is SiO 2) there are Si and O atoms. Therefore, the bonds covalent polar.

Physical properties of substances with an atomic crystal lattice:

1. strength, hardness
2. high melting points (refractoriness)
3. non-volatile substances
4. insoluble (neither in water nor in other solvents)

All these properties are due to the strength covalent bonds.

There are few substances in an atomic crystal lattice. There is no particular pattern, so you just need to remember them:

1. Allotropic modifications of carbon (C): diamond, graphite.
2. Boron (B), silicon (Si), germanium (Ge).
3. Only two allotropic modifications of phosphorus have an atomic crystal lattice: red phosphorus and black phosphorus. (white phosphorus has a molecular crystal lattice).
4. SiC – carborundum (silicon carbide).
5. BN – boron nitride.
6. Silica, rock crystal, quartz, river sand - all these substances have the composition SiO 2.
7. Corundum, ruby, sapphire - these substances have the composition Al 2 O 3.

Surely the question arises: C is both diamond and graphite. But they are completely different: graphite is opaque, stains, and conducts electricity, while diamond is transparent, does not stain, and does not conduct electricity. They differ in structure.

Both are atomic lattice, but different. Therefore, the properties are different.

Ionic crystal lattice.

Classic example: table salt: NaCl. At the lattice nodes there are individual ions: Na + and Cl – . The lattice is held in place by electrostatic forces of attraction between ions (“plus” is attracted to “minus”), that is ionic bond.

Ionic crystal lattices are quite strong, but fragile; the melting temperatures of such substances are quite high (higher than those of representatives of metallic lattices, but lower than those of substances with an atomic lattice). Many are soluble in water.

As a rule, there are no problems with determining the ionic crystal lattice: where there is an ionic bond, there is an ionic crystal lattice. This: all salts, metal oxides, alkalis(and other basic hydroxides).

Metal crystal lattice.

The metal grating is sold in simple substances metals. We said earlier that all the splendor of the metallic bond can only be understood in conjunction with the metallic crystal lattice. The hour has come.

The main property of metals: electrons on external energy level They are poorly held, so they are easily given away. Having lost an electron, the metal turns into a positively charged ion - a cation:

Na 0 – 1e → Na +

In a metal crystal lattice, the processes of electron release and gain constantly occur: an electron is detached from a metal atom at one lattice site. A cation is formed. The detached electron is attracted by another cation (or the same one): a neutral atom is formed again.

The nodes of a metal crystal lattice contain both neutral atoms and metal cations. And free electrons travel between the nodes:

These free electrons are called electron gas. They determine the physical properties of simple metal substances:

1. thermal and electrical conductivity
2. metallic shine
3. malleability, ductility

This is a metallic bond: metal cations are attracted to neutral atoms and free electrons “glue” it all together.

How to determine the type of crystal lattice.

P.S. There's something in school curriculum And Unified State Exam program on this topic something we don't entirely agree with. Namely: the generalization that any metal-nonmetal bond is an ionic bond. This assumption was deliberately made, apparently to simplify the program. But this leads to distortion. The boundary between ionic and covalent bonds is arbitrary. Each bond has its own percentage of “ionicity” and “covalency”. The bond with a low-active metal has a small percentage of “ionicity”; it is more like a covalent one. But according to the Unified State Exam program, it is “rounded” towards the ionic one. This gives rise to sometimes absurd things. For example, Al 2 O 3 is a substance with an atomic crystal lattice. What kind of ionicity can we talk about here? Only a covalent bond can hold atoms together in this way. But according to the metal-non-metal standard, we classify this bond as ionic. And we get a contradiction: the lattice is atomic, but the bond is ionic. This is what oversimplification leads to.

Details Category: Molecular-kinetic theory Published 11/14/2014 17:19 Views: 14761

In solids, particles (molecules, atoms and ions) are located so close to each other that the interaction forces between them do not allow them to fly apart. These particles can only perform oscillatory movements around the equilibrium position. Therefore, solids retain their shape and volume.

Based on their molecular structure, solids are divided into crystalline And amorphous .

Structure of crystalline bodies

Crystal lattice

Crystalline are those solids, molecules, atoms or ions in which they are arranged in a strictly defined geometric order, forming a structure in space called crystal lattice . This order is periodically repeated in all directions in three-dimensional space. It persists over long distances and is not limited in space. They call him in a long way .

Types of crystal lattices

A crystal lattice is a mathematical model that can be used to imagine how particles are arranged in a crystal. Mentally connecting the points in space where these particles are located with straight lines, we get a crystal lattice.

The distance between atoms located at the sites of this lattice is called lattice parameter .

Depending on which particles are located at the nodes, crystal lattices are molecular, atomic, ionic and metallic .

The properties of crystalline bodies such as melting point, elasticity, and strength depend on the type of crystal lattice.

When the temperature rises to a value at which the melting of a solid begins, the crystal lattice is destroyed. The molecules gain more freedom, and the solid crystalline substance passes into the liquid stage. The stronger the bonds between molecules, the higher the melting point.

Molecular lattice

In molecular lattices, the bonds between molecules are not strong. Therefore, under normal conditions, such substances are in a liquid or gaseous state. The solid state is possible for them only at low temperatures. Their melting point (transition from solid to liquid) is also low. And under normal conditions they are in a gaseous state. Examples are iodine (I 2), “dry ice” (carbon dioxide CO 2).

Atomic lattice

In substances that have an atomic crystal lattice, the bonds between atoms are strong. Therefore, the substances themselves are very hard. They melt at high temperatures. Silicon, germanium, boron, quartz, oxides of some metals, and the hardest substance in nature, diamond, have a crystalline atomic lattice.

Ionic lattice

Substances with ionic crystal lattice include alkalis, most salts, and oxides of typical metals. Since the attractive force of ions is very strong, these substances can melt only at very high temperatures. They are called refractory. They have high strength and hardness.

Metal grill

At the nodes of the metal lattice, which all metals and their alloys have, both atoms and ions are located. Thanks to this structure, metals have good malleability and ductility, high thermal and electrical conductivity.

Most often, the crystal shape is a regular polyhedron. The faces and edges of such polyhedra always remain constant for a particular substance.

A single crystal is called single crystal . It has a regular geometric shape, a continuous crystal lattice.

Examples of natural single crystals are diamond, ruby, rock crystal, rock salt, Iceland spar, quartz. IN artificial conditions Single crystals are obtained during the crystallization process, when, by cooling solutions or melts to a certain temperature, a solid substance in the form of crystals is isolated from them. With a slow crystallization rate, the cut of such crystals has a natural shape. In this way, under special industrial conditions, single crystals of semiconductors or dielectrics are obtained.

Small crystals randomly fused together are called polycrystals . The clearest example of a polycrystal is granite stone. All metals are also polycrystalline.

Anisotropy of crystalline bodies

In crystals, particles are located with different densities in different directions. If we connect atoms in one of the directions of the crystal lattice with a straight line, then the distance between them will be the same throughout this direction. In any other direction, the distance between the atoms is also constant, but its value may already differ from the distance in the previous case. This means that interaction forces of different magnitudes act between atoms in different directions. Therefore, the physical properties of the substance in these directions will also differ. This phenomenon is called anisotropy - dependence of the properties of matter on direction.

Electrical conductivity, thermal conductivity, elasticity, refractive index and other properties of a crystalline substance vary depending on the direction in the crystal. Electric current is conducted differently in different directions, the substance is heated differently, and light rays are refracted differently.

In polycrystals the phenomenon of anisotropy is not observed. The properties of the substance remain the same in all directions.

When performing many physical and chemical reactions the substance passes into a solid state of aggregation. In this case, molecules and atoms tend to arrange themselves in such a spatial order in which the forces of interaction between particles of matter would be maximally balanced. This is how the strength of the solid substance is achieved. Atoms, once occupying a certain position, perform small oscillatory movements, the amplitude of which depends on temperature, but their position in space remains fixed. The forces of attraction and repulsion balance each other at a certain distance.

Modern ideas about the structure of matter

Modern science states that an atom consists of a charged nucleus, which carries a positive charge, and electrons, which carry negative charges. At a speed of several thousand trillion revolutions per second, electrons rotate in their orbits, creating an electron cloud around the nucleus. The positive charge of the nucleus is numerically equal to the negative charge of the electrons. Thus, the atom of the substance remains electrically neutral. Possible interactions with other atoms occur when electrons are detached from their parent atom, thereby disturbing the electrical balance. In one case, the atoms are arranged in a certain order, which is called a crystal lattice. In another, due to the complex interaction of nuclei and electrons, they combine into molecules various types and complexity.

Definition of crystal lattice

Taken together, various types of crystalline lattices of substances are networks with different spatial orientations, at the nodes of which ions, molecules or atoms are located. This stable geometric spatial position is called the crystal lattice of the substance. The distance between the nodes of one crystal cell is called the identity period. The spatial angles at which the cell nodes are located are called parameters. According to the method of constructing bonds, crystal lattices can be simple, base-centered, face-centered, and body-centered. If the particles of matter are located only in the corners of the parallelepiped, such a lattice is called simple. An example of such a lattice is shown below:

If, in addition to the nodes, the particles of the substance are located in the middle of the spatial diagonals, then this arrangement of particles in the substance is called a body-centered crystal lattice. This type is clearly shown in the figure.

If, in addition to the nodes at the vertices of the lattice, there is a node at the place where the imaginary diagonals of the parallelepiped intersect, then you have a face-centered type of lattice.

Types of crystal lattices

The different microparticles that make up a substance determine the different types of crystal lattices. They can determine the principle of building connections between microparticles inside a crystal. Physical types of crystal lattices are ionic, atomic and molecular. This also includes various types of metal crystal lattices. Studying the principles internal structure Chemistry deals with elements. The types of crystal lattices are presented in more detail below.

Ionic crystal lattices

These types of crystal lattices are present in compounds with an ionic type of bond. In this case, lattice sites contain ions with opposite electrical charges. Thanks to electromagnetic field, the forces of interionic interaction turn out to be quite strong, and this determines the physical properties of the substance. Common characteristics are refractoriness, density, hardness and the ability to conduct electric current. Ionic types of crystal lattices are found in substances such as table salt, potassium nitrate and others.

Atomic crystal lattices

This type of structure of matter is inherent in elements whose structure is determined by covalent chemical bonds. Types of crystal lattices of this kind contain individual atoms at the nodes, connected to each other by strong covalent bonds. This type of bond occurs when two identical atoms “share” electrons, thereby forming a common pair of electrons for neighboring atoms. Thanks to this interaction, covalent bonds bind atoms evenly and strongly in a certain order. Chemical elements that contain atomic types of crystal lattices are hard, have a high melting point, are poor conductors of electricity, and are chemically inactive. Classic examples of elements with a similar internal structure include diamond, silicon, germanium, and boron.

Molecular crystal lattices

Substances that have a molecular type of crystal lattice are a system of stable, interacting, closely packed molecules that are located at the nodes of the crystal lattice. In such compounds, the molecules retain their spatial position in the gaseous, liquid and solid phases. At the nodes of the crystal, molecules are held together by weak van der Waals forces, which are tens of times weaker than the forces of ionic interaction.

The molecules that form a crystal can be either polar or nonpolar. Due to the spontaneous movement of electrons and vibrations of nuclei in molecules, the electrical equilibrium can shift - this is how an instantaneous electric dipole moment arises. Appropriately oriented dipoles create attractive forces in the lattice. Carbon dioxide and paraffin are typical examples of elements with a molecular crystal lattice.

Metal crystal lattices

A metal bond is more flexible and ductile than an ionic bond, although it may seem that both are based on the same principle. The types of crystal lattices of metals explain their typical properties - such as mechanical strength, thermal and electrical conductivity, and fusibility.

A distinctive feature of a metal crystal lattice is the presence of positively charged metal ions (cations) at the sites of this lattice. Between the nodes there are electrons that are directly involved in creating an electric field around the lattice. The number of electrons moving around within this crystal lattice is called electron gas.

In the absence of an electric field, free electrons perform chaotic motion, randomly interacting with lattice ions. Each such interaction changes the momentum and direction of motion of the negatively charged particle. With their electric field, electrons attract cations to themselves, balancing their mutual repulsion. Although electrons are considered free, their energy is not enough to leave the crystal lattice, so these charged particles are constantly within its boundaries.

The presence of an electric field gives the electron gas additional energy. The connection with ions in the crystal lattice of metals is not strong, so electrons easily leave its boundaries. Electrons move along lines of force, leaving behind positively charged ions.

Conclusions

Chemistry attaches great importance to the study of the internal structure of matter. The types of crystal lattices of various elements determine almost the entire range of their properties. By influencing crystals and changing their internal structure, it is possible to enhance the desired properties of a substance and remove unwanted ones and transform chemical elements. Thus, studying the internal structure of the surrounding world can help to understand the essence and principles of the structure of the universe.

5. Ionic and metallic bond. Hydrogen bond. Valence

5.4. Types of crystal lattices

Substances in the solid state can have an amorphous and crystalline structure. In amorphous substances (glass, polymers) the arrangement of particles is disordered, but in crystalline substances the structural units (atoms, molecules or ions) are arranged in a strict order.

Under crystal lattice refers to the framework that is formed if the structural units of a crystal are connected by imaginary straight lines. The points of intersection of these lines are called crystal lattice nodes. Depending on the nature of the particles located at the nodes of the crystal lattice, as well as on the type chemical bond between them there are four main types (types) of crystal lattices: atomic, molecular, ionic and metallic.

Substances with atomic, ionic and metal crystal lattices have a non-molecular structure

In nodes atomic crystal lattice there are atoms of the same or different chemical elements(usually non-metals) linked together by strong covalent bonds (see Fig. 16.1 on p. 347). Substances with an atomic lattice are called atomic or covalent crystals.

Let us remember substances with an atomic crystal lattice: boron, silicon, diamond, graphite, black and red phosphorus, carborundum SiC, silicon oxide (IV) SiO 2.

Due to the high energy of covalent bonds, substances of atomic structure have a very high melting point, high hardness and strength, and low solubility; as a rule, they are dielectrics or semiconductors (silicon, germanium). The hardest natural substance is diamond (melting point 3500 °C), the most refractory is graphite (3700 °C); carborundum SiC (2700 °C) and silica SiO 2 (1610 °C) have a high melting point.

In nodes molecular crystals(substances with a molecular crystal lattice, molecular structure) there are molecules (Fig. 5.7, a). Molecules are connected to each other by weak intermolecular forces (do not be confused: in molecules the bond is covalent, i.e. strong), which requires relatively little energy to break. Therefore, molecular substances have low strength, low hardness, significant compressibility, and low melting and boiling points. They are characterized by volatility, many have an odor, and some sublime. Molecular crystals do not conduct electricity and can be soluble in polar and non-polar solvents.

Most substances with covalent polar or nonpolar bonds have a molecular crystal lattice, with the exception of the atomic structure substances listed above. The molecular structure is more typical for organic matter. Examples of substances of molecular structure: noble gases (for them the concepts of atom and molecule are identical, we can say that noble gases consist of monatomic molecules), halogens (in the solid state), white phosphorus P4, orthorhombic and monoclinic sulfur S8, solid oxygen, ozone, nitrogen, water, hydrogen halides, alkanes, benzene.

Rice. 5.7. The structure of the crystal lattice of carbon dioxide (CO 2) in the solid state (a) and sodium chloride (b)

All substances with ionic bonds form ionic crystal lattices, have an ionic structure. These are salts, basic and amphoteric oxides, bases, binary compounds metals with non-metals (hydrides, nitrides, etc.). At the nodes of ionic crystals there are oppositely charged simple or complex cations and anions, interconnected by a strong ionic bond (Fig. 5.7, b). Due to the strength of the ionic bond, ionic crystals have great hardness, are non-volatile and odorless, and are characterized by high boiling points and melting. At room temperature, ionic substances conduct electric current and heat poorly; many are highly soluble in polar solvents; their aqueous solutions and melts conduct electric current (electrolytes). Ionic substances are characterized by weak deformability and fragility, since when ions are displaced relative to each other, repulsive forces arise between similarly charged ions.

Substances with a metal bond form metal crystal lattices(metal crystals), in which (see Fig. 5.1) communication is provided by free electrons (electron gas).

For this reason, simple substances metals (and their alloys) have a characteristic metallic luster, very high thermal and electrical conductivity, they are opaque, malleable and ductile. Metals have a wide range of melting points (for example, under normal conditions, mercury is in a liquid aggregate state), hardness (soft lead and very hard chromium), which is due to some differences in the nature of the metallic bond of different metals. As already noted, the melting temperature of metals can serve as a measure of the strength of a metallic bond: the higher tmelt, the greater the energy of the metallic bond. The melting point of metals increases in the series:

mercury → alkali metals → alkaline earth metals →

→ d-family metals → tungsten.

Example 5.4. Among chlorine compounds with elements of the 3rd period, the lowest melting point is:

Solution. The substance we are looking for is SCl 2, since it has a molecular crystal lattice (all other substances are ionic).